1. Field of the Invention
The present invention relates to an optical disk drive and, more particularly, to a demodulator for demodulating an auxiliary recording signal superposed on an optical disk and a semiconductor integrated circuit monolithically integrating the demodulator on a single semiconductor chip.
2. Description of the Related Art
A compact disk-recordable/rewritable (CD-R/RW) device is available as a recordable optical disk. A guiding groove is preformatted on the CD-R/RW device to guide a pickup. The guiding groove meanders slightly in a radial direction so as to detect a clock for rotation control. Such a track structure is referred to as a “wobbled land groove”. Information superposed on a wobbled track of the CD-R/RW device is known as an absolute time in pregroove (ATIP). The absolute time on the CD-R/RW device can be obtained from the ATIP signal. A code sequence of an ATIP signal is converted into a digital frequency modulation signal, i.e., a biphase signal, and further subjected to frequency shift keying (FSK) modulation to be recorded as a wobble track.
A digital versatile disk+recordable/rewritable (DVD+RW) device is also available as a recordable optical disk. As in the case of the CD-R/RW device, a track structure of the DVD+RW device is the wobbled land groove. Information superposed on a wobbled track of the DVD+RW is known as an address in pregroove (ADIP). A code sequence of an ADIP signal is subjected to phase shift keying (PSK) modulation. A physical address on the DVD+RW device is obtained from the ADIP signal to be used for positioning of a recording area or verification of recording position during recording. The ATIP and ADIP signals are very important signals for detecting a state of the optical disk during reproducing and recording.
The ATIP signal is demodulated by an ATIP demodulator from a wobble signal detected from the optical disk by the pickup. Herein, the “wobble signal” is a signal obtained by the calculation of A−B, where A and B denote signals obtained from A and B sections of an optical detector in the pickup. The ATIP demodulator subjects the wobble signal to FSK demodulation, and generates a binarized wobble signal. The binarized wobble signal is latched in synchronization with a clock to demodulate the biphase signal. A biphase demodulator demodulates the ATIP signal from the biphase signal. The ADIP demodulator which demodulates the ADIP signal from the wobble signal first binarizes the wobble signal. The binarized wobble signal is subjected to EX-OR calculation together with a clock. A result of the EX-OR calculation is latched in synchronization with the clock to demodulate the ADIP signal.
During play-back of a recorded disk, since a recording signal is superposed on the wobble signal, a signal to noise (S/N) ratio of the wobble signal is relatively decreased. Furthermore, during recording on the optical disk, a recording light is reflected and superposed on the wobble signal to significantly decrease the S/N ratio of the wobble signal.
The ATIP demodulator cannot perform normal FSK demodulation of a signal portion containing a distorted waveform when the waveform of the wobble signal is distorted by decreasing the S/N ratio. Consequently, a demodulation error occurs in the demodulated ATIP signal. In the ADIP demodulator, when the wobble signal is distorted, a signal portion containing the distorted waveform cannot be subjected to normal binarization. Consequently, the demodulated ADIP signal has a demodulation error. As described above, when the S/N ratio of the wobble signal is significantly decreased, it is difficult for the ATIP demodulator and the ADIP demodulator to carry out accurate demodulation.